It is known in the prior art to produce light at a source and then transport the light by a light guide to a distant location where the light is used. Light guides generally have the form of a hollow structure with a reflective surface formed along an inner wall of the guide. By generating light in the hollow cavity, the light is transported by reflection along the inner wall. The light is delivered by aiming the guide at the target area, or opening a hole in the wall and reflecting light out to the target area. An example is provided by U.S. Pat. No. 4,459,642 to Kei Mori for an Optical Lighting Device. The example shows multiple coaxial tubes having reflective internal surfaces, and numerous openings along the tubes to release the light guided by internal reflections. There are several advantages to distributing light by a light guide. A single efficient source may be used instead of numerous less efficient sources. Fewer sources means fewer electrical connections, and less related electrical equipment.
Two important conditions affect efficient transport in a light guide. The source should produce light in a narrow beam, which maye taken as a beam half-angle of a few degrees. A broad beam has light with a small angle of incidence, which results in multiple reflections and therefore a large light loss, since there is a loss occurring at each reflection. A narrow beam, directed axially in the guide, results in fewer reflections, each having a high angle of incidence called a grazing angle. The preferred beam is then narrow, generally parallels the guide walls to forming grazing reflections, and therefore has a few reflections as possible.
A second influential aspect is the reflectivity of the walls. Since a loss occurs with each reflection, a high reflectivity at grazing angles is importanat. Unfortunately, metallic reflectors do not have high reflectance at grazing angles. Most metallized films, and even polished anodized aluminum, have reflectances of less than 95% at perpendicular incidence; and at grazing incidence, the reflectance percentage drops even lower, to 70 or 80 percent. Enhanced reflectors are known, in which a metallized film is overcoated with a transparent layer having a precise thickness relative to a particular wavelength of light. The film over metal light guides can achieve reflectances of greater than 95% for normal incidence. While it is theoretically possible to further enhance reflectance for grazing angles, the enhancement is normally limited to only one color, and one angle of incidence. The enhanced film on metal light guide is then much less effective for the range of grazing angles produced by most radiating sources, and also for the spectrum of white light normally produced and desired. There is then a need to provide an enhanced reflector effective over a range of grazing angles, and there is a need to provide a light guide with enhanced reflectivity for a broad range of wave lengths. There is a further need for a wall material for use in light guide systems having high reflectance at grazing angles that is economical and adaptable to simple manufacturing processes.